JP-A-08-295140 describes a hybrid vehicle in which torques of an engine and a first motor is delivered to driving wheels. The hybrid vehicle is provided with a differential gear unit including a first rotary element connected to an output shaft through a clutch, a second rotary element connected to the first motor, and a third rotary element connected to the driving wheels, and further provided with a second motor generating a torque delivered to the driving wheels from downstream side of the third rotary element. In the hybrid vehicle taught by JP-A-08-295140, a drive mode can be selected from a hybrid mode in which the clutch is brought into engagement to transmit torques of the engine and the first motor to the driving wheels, and an EV mode in which the clutch is brought into disengagement to transmit only torque of the second motor to the driving wheels.
Control systems for the hybrid vehicle thus having a clutch for selectively interrupting a torque transmission between the prime mover and the driving wheels are also described in JP-A-2012-081811, JP-A-2011-230706, JP-A-2010-188776 and JP-A-2009-274566 respectively. For example, JP-A-2012-081811 describes a control device configured to inhibit the EV running in which the driving wheels are rotated only by power of the motor, in case torques of the engine and the motor cannot be interrupted due to failure so as to avoid occurrence of secondary failure.
JP-A-2011-230706 describes a hybrid vehicle in which a clutch is disposed between an engine and a motor, and a gear train connects an output shaft of the motor and the driving wheels in series. In the hybrid vehicle, the power of the engine is transmitted to the motor so that the motor serves as a generator. According to the teachings of JP-A-2011-230706, therefore, the control device is configured to reduce a discharging amount of a battery while increasing a threshold value for determining a reduction in an SOC level, so as to prevent an excessive reduction in the SOC level in case torques of the engine and the motor cannot be interrupted by failure.
JP-A-2010-188776 describes a hybrid vehicle, in which a motor is connected to an output shaft of an engine, and a transmission is connected to an output shaft of the motor. In the hybrid vehicle of this kind, if the vehicle is powered only by the motor under a situation that a torque transmission of the engine and the motor cannot be interrupted due to failure, a power loss will be caused to rotate the engine passively. That is, the motor is required to generate a large power to keep propelling the vehicle. As a result, an electric consumption is increased and hence a cruising distance in a motor running mode is shortened. According to the teachings of JP-A-2010-188776, therefore, the engine is activated even if a condition of motor running mode is satisfied so as to reduce a power loss to rotate the engine passively, under a situation that the torque transmission of the engine and the motor cannot be interrupted due to failure. In this case, if the SOC of the battery is sufficient, the vehicle is allowed to be powered only by the motor without activating the engine.
JP-A-2009-274566 also describes a hybrid vehicle in which an output shaft of an engine is connected to a generator, an output shaft of the generator is connected to driving wheels through a clutch, and a motor to which power is applied from the generator is connected to an output side of the clutch. According to the teachings of JP-A-2009-274566, a so-called “series mode” in which a power of the engine is converted into electric power by the generator to power the vehicle only by the motor is inhibited when the clutch cannot be brought into disengagement by failure. By contrast, when the clutch cannot be brought into engaging completely while causing slip by failure, the vehicle is powered only by the motor without activating the engine.
JP-A-2013-001182 also describes a power transmission control device for a hybrid vehicle in which an output shaft of the engine is connected to an input shaft of the transmission trough a manual clutch, and an output shaft of the transmission is connected to a motor. According to the teachings of JP-A-2013-001182, the EV mode is inhibited or an output torque of the motor is restricted in the event of failure of a sensor of a clutch pedal.